In vitro growth of hepatitis A virus was first accomplished with virus inocula prepared by prior in vivo passage of hepatitis A virus in certain sub-human primates, Provost et al., U.S. Pat. No. 4,164,566. Subsequently, a procedure was described for the growth of hepatitis A virus in vitro by direct inoculation of cell cultures with human clinical specimens containing the hepatitis A virus, Provost et al., European patent application No. 025745. Hepatitis A virus isolates may be initiated to in vitro growth in cell culture by either of the described techniques. The in vitro cultured hepatitis A virus can then be adapted to growth in several types of cell cultures by serial passages. Such adaptive passaging results in increased hepatitis A virus yields which are absolutely necessary for the development of inactivated viral vaccines. In vitro growth of hepatitis A virus has allowed the adaptation of the virus to growth in cell lines which are acceptable for the production of vaccines, e.g., human fetal diploid lung cells and MRC-5 cells. The MRC-5 cells are suitable for the production of live hepatitis A virus vaccine but are less suitable for inactivated vaccine production. The Vero cell line is more advantageous for the growth of virus in the quantities necessary for inactivated vaccine production. Vero cells are transformed but non-tumorigenic cells derived from cercopithicus monkey kidney, Yasumura et al., Nippon Rinsko 21:1201-1215 (1963). Vero cells are available in larger quantities than are MRC-5 cells and are more readily adaptable to large scale cell culture techniques than are MRC-5 cells.
Attempts to grow hepatitis A virus in Vero cell cultures under conventional growth conditions, e.g., cultivation at about 37.degree. C., have resulted in little or no production of virus particles, Locarnini et al., J. Virology 37:216-225 (1981). The ability to propagate hepatitis A virus in Vero cells would greatly enhance the development of an inactivated vaccine since Vero cells are available in sufficient quantity for mass culture, are considered an acceptable cell substrate for inactivated vaccine production and are technologically advantageous when compared with other acceptable cell lines.